The private key is only known to the related participant and must always be kept private, as it allows complete control over the associated account. A loss would mean that the access to the account, and therefore to the assets, is lost. The private key is simply a number randomly selected by the system and assigned to a participant. In the Bitcoin example it is a number of 256 bits. For simplicity, the binary number is converted and displayed as a hexadecimal number of 64 characters .
If something has been encrypted with the public key, it can only be decrypted with the corresponding private key. Vice versa, a signature can be created with the private key and everyone can verify the authenticity of the signature with the corresponding public key. This way, Bitcoin can ensure that only the account holder can carry out transactions .
 Antonopoulos, A. (2015) Mastering Bitcoin Unlocking Digital Cryptocurrencies